Researchers can make rodents forget their fears by encouraging the development of new neurons. | AAAS/Carla Schaffer

Why is it so hard to recall memories from early childhood? A new study suggests that the birth of new neurons could play a significant role in this "infantile amnesia," which seems to occur across a wide range of species.

New neurons are continually integrated into the brain's hippocampus throughout one's life, but the process is especially rapid during infancy. Many scientists have wondered if this constant neurogenesis might rearrange connections in the brain, destabilizing old memories in the process.

Katherine Akers from The Hospital for Sick Children in Toronto, along with colleagues from the University of Toronto and Fujita Health University in Toyoake, Japan, used mice, guinea pigs and little rodents called degus to explore these potential correlations between the generation of new neurons and forgetfulness.

"About 20 years ago, researchers clearly established that neurogenesis was happening in the hippocampi of adult rodents and humans, and ever since then, we've been curious," explained Paul Frankland, a co-author of the Science report. "Traditionally, researchers have manipulated neurogenesis in the hippocampi of their test animals and then trained the animals in hippocampus-related tasks a week or two later, finding that increased neurogenesis can lead to improvements in learning."

"But we asked the opposite question: How does the integration of neurons affect memories that are already there?"

Akers and her team first used mild electric shocks to train a group of mice to fear a particular environment. That way, whenever the researchers reintroduced the mice to that environment, they could gauge how well the mice remembered it. (Mice that associate the environment with electric shocks freeze in place for a certain amount of time.) The researchers then gave some of those trained mice access to running wheels, since exercise in general has been shown to boost neurogenesis.

A couple weeks later, the researchers returned their mice to the environment the rodents had been trained to fear. They found that the mice with running wheels had largely forgotten their fear of the place. But mice without running wheels seemed to recall the recent electric shocks quite well, the researchers say.

Akers and the other researchers also used a drug to slow the rate of neurogenesis in infant mice and discovered that those mice were better at retaining memories than their untreated infant counterparts.

Finally, the researchers tested the effects of neurogenesis on forgetfulness in guinea pigs and degus. "These animals have longer gestation periods, so at birth they're more neurologically mature than mice and rats," according to Frankland. They found that infant guinea pigs and degus don't forget their fears as quickly as infant mice. But, when the researchers gave them a drug to stimulate neurogenesis, these more mature infants started experiencing infantile amnesia too.

Taken together, the findings suggest that forgetting certain things might be an essential aspect of forming new memories.

"It's still speculative at this point, but we think that it's time to embrace the idea that forgetting is a good thing," Frankland concluded. "You don't want to fill up on useless memories or mix up old memories with new ones...You don't want to remember everything; just a subset of things."

Akers and her team suggest that memories that are activated and recalled frequently become consolidated as long-term memories, while smaller, more mundane details might be forfeit to our ever-evolving neural pathways. But it will take more research to clarify whether forgetting really helps memory work more efficiently.